Communication systems designed to incorporate the characteristic of communicating with many remote subscriber units for brief intervals on the same communication channel are termed multiple access communication systems. One type of communication system which can be a multiple access system is a spread spectrum system. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Generally, three types of spread spectrum communication techniques exist, including:
Direct Sequence
The modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are referred to as "direct sequence" modulated systems.
Hopping
Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. These systems are called "frequency hoppers." The transmitter jumps from frequency to frequency within some predetermined set; the order of frequency usage is determined by a code sequence. Similarly "time hopping" and "time-frequency hopping" have times of transmission which are regulated by a code sequence.
Chirp
Pulse-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
Information (i.e. the message signal) can be embedded in the spread spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code, because the combination of the spreading code and the information, typically a binary code, involves module-2 addition. Alternatively, the information or message signal may be used to modulate a carder before spreading it.
Thus, a spread spectrum system must have two properties: (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth.
Spread spectrum communication systems can be implemented as multiple access systems in a number of different ways. One type of multiple access spread spectrum system is a code division multiple access (CDMA) system. CDMA spread spectrum systems may use direct sequence (DSCDMA) or frequency hopping (FH-CDMA) spectrum spreading techniques. FH-CDMA systems can further be divided into slow frequency hopping (SFH-CDMA) and fast frequency hopping (FFH-CDMA) systems. In SFH-CDMA systems, several data symbols representing a sequence of data bits to be transmitted modulate the carrier wave within a single hop; in FFH-CDMA systems, the carrier wave hops several times per data symbol.
In a SFH-CDMA system, multiple communication channels are accommodated by the assignment of portions of a broad frequency band to each particular channel. For example, communication between two communication units in a particular communication channel is accomplished by using a frequency synthesizer to generate a carrier wave in a particular portion of a predetermined broad frequency band for a brief period of time. The frequency synthesizer uses an input spreading code to determine the particular frequency from within the set of frequencies in the broad frequency band at which to generate the carrier wave. Spreading codes are input to the frequency synthesizer by a spreading code generator. The spreading code generator is periodically clocked or stepped through different transitions which causes different or shifted spreading codes to be output to the frequency synthesizer. Therefore, as the spreading code generator is periodically clocked, the carrier wave is frequency hopped or reassigned to different portions of the frequency band. In addition to hopping, the carrier wave is modulated by data symbols representing a sequence of data bits to be transmitted. A common type of carrier wave modulation used in SFH-CDMA systems is M-ary frequency shift keying (MFSK), where k=log .sub.2 M data symbols are used to determine which one of the M frequencies is to be transmitted.
Multiple communication channels are allocated by using a plurality of spreading codes to assign portions of the frequency band to different channels during the same time period. As a result, transmitted signals are in the same broad frequency band of the communication channel, but within unique portions of the broad frequency band assigned by the unique spreading codes. These unique spreading codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. Particular transmitted signals can be retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Further, when the spreading codes are orthogonal to one another, the received signal can be correlated with a particular spreading code such that only the desired signal related to the particular spreading code is enhanced while the other signals are not enhanced.
As CDMA technology becomes incorporated into next-generation cellular systems, practical system complications arise. Cellular systems typically require that a signal strength measurement for handoff between cell sites be performed as the handoff criterion. As a mobile moves away from a serving cell site, the signal strength decreases to a point where signal quality degrades and handoff to a neighboring cell site is necessary. To enhance the handoff process, cellular systems incorporate mobile assisted handoff (MAHO) rather than leave the entire handoff procedure to be performed by the cell site. The use of MAHO eliminates the need for scan receivers at the cell site, reduces the amount of inter-cell communications and allows all measurements to be made by a single measurement device, thus reducing calibration requirements at the cell sites. More importantly, in the SFH-CDMA systems, an adjacent cell may not be able to measure the signal strength of a mobile because of other mobiles transmitting on the frequency of the target mobile.
The simplest method of implementing MAHO is to have the mobile scan the signalling channels in use in adjacent cells. For example, if a cell were sectored, the serving cell would transmit to the mobile the channel numbers of the Forward Control Channel (FOCC) in each of the six adjacent cells. If the signal strength of an adjacent cell exceeded that of the serving cell as measured by the mobile, a handoff to the stronger cell would be attempted by the serving cell site.
This approach, however, has a number of drawbacks. First, the FOCC is very probably being broadcast on an OMNI antenna, while the voice channels (the channels whose signal strength are really of interest) are likely to be transmitted per sector. The quality of the OMNI channel will probably not be a good reflection of the quality of the best sectored potential voice channel. In addition, if the cell site determines that a handoff is necessary, it will have to make an educated guess as to which sector in the new cell will be the best server. This could be accomplished based on the relative geographical position of the two cell sites in the physical location of the mobile; however, intervening terrain could render the decision incorrect. This could lead to drop calls, rather than just avoidance of the handoff. Use of a voice channel in each potential handoff sector would clearly be better than monitoring the FOCC, but this is not possible since the voice channels in adjacent sectors are frequency-hopping with a different hopping pattern than the serving cell.
Thus, a need exists for a frequency-hopping communications system which supports the use and implementation of MAHO.